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# Valve Authority

## The valve authority expresses the ratio between pressure drop across the control valve compared to total pressure drop across the whole system.

Control valve authority can be expressed as

N = dpv / (dpv + dpc)                          (1)

where

N = valve authority

dpv = pressure drop across a full open control valve (psi, Pa, kPa, bar)

dpc = pressure drop across the circuit (psi, Pa, kPa, bar)

To obtain control it is recommended that the control valve size is selected so that its authority is never less than 0.5. In general valve authority between

• 0 - 0.25 -> unstable to fair control
• 0.25 - 0.5 -> fair to good control
• 0.5 - 1.0 -> good control

### Example - Control Valve Authority in a Heating System

The total pressure drop in a heating system circuit at full load is 15 kPa. The volume flow is 1.4 l/s.

For decent control the valve authority is set to N = 0.8. The pressure drop across the control valve dpv can be calculated by transforming (1) to

dpv = N dpc / (1 - N)                     (1b)

- or with values

dpv = 0.8 (15 kPa)  / (1 - 0.8)

= 60 kPa

The flow factor for the valve with volume flow 1.4 l/s can be estimated to approximately

Kv = 7

by using this flow factor diagram. For a Straight Through Diaphragm Valve of "Saunders" type this corresponds to a 15 mm valve.

## Related Topics

• ### Control Valve Sizing

Control Valve terminology, bodies, trim, flow characteristics, Cv and Kv sizing, noise, actuators, positioners and more.

## Related Documents

• ### API 6D Valves - Pressure Ratings vs. Temperature

Temperature and pressure ratings for valves according API 6D.
• ### ASTM A216 Valves - Pressure Ratings vs. Temperature

Pressure and temperature ratings for cast carbon steel valves.
• ### Ball Valves - Flow Coefficients Cv

Flow coefficients - Cv - for typical ball valves - reduced and full bore.
• ### Control Valves - Cavitation

Control valves and cavitation, application ratio and multi stage control valves.
• ### Control Valves - Flow Characteristics

Flow capacities vs. stem opening.
• ### Liquid, Steam and Gas - Flow Factor Kv

Calculate flow factor Kv for liquids, steam and gases - metric units.
• ### Steam Control Valves - Calculate Flow Factor Kv

Designing steam control valves and their Kv values.
• ### Straight Through Diaphragm Valves - Flow Coefficients

Flow coefficients - Cv - and flow factors - Kv - for straight through diaphragm valves.
• ### Valve Selection Guide

Applications guide for selecting valves.
• ### Valves - Specific Services

In case of a specific service - the choose of a valve may be simplified by following an established practice.
• ### Valves - Typical Operating Temperatures

Operating temperatures for typical categories of valves - ball valves, butterfly valves and more.
• ### Valves Types - Operating Size Ranges

Types of valves and their typical operating sizes.
• ### Water Control Valves - Flow Factor Kv Diagram

Sizing of water control valves - SI units
• ### Water Control Valves - Flow Coefficient Cv Diagram

Sizing of water control valves - Imperial units.
• ### Water Control Valves - Kv Calculation

Designing water process control valves and their Kv values.

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## Citation

• The Engineering ToolBox (2008). Valve Authority. [online] Available at: https://www.engineeringtoolbox.com/control-valve-authority-d_1066.html [Accessed Day Month Year].

Modify the access date according your visit.

6.3.17

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